1. Field of the Invention
This invention relates to circuits for power supply, and more particularly to a circuit suitable for use in power supply for a telephone set and the like to which a direct current (DC) voltage having superposed thereon alternating current (AC) signals is given.
2. Description of the Prior Art
In general, delivered to a telephone set through a telephone line from a central telephone exchange is an input having superposed thereon AC signals such as voices, etc., and moreover, the input signal voltages depend upon the value of distance between the central telephone exchange and the telephone set. For this reason, when various electronic circuits of the telephone set are composed of semiconductive integrated circuits (IC), it is necessary to stabilize a driving input delivered to the IC in order to drive the IC reliably.
In the same manner as other electronic circuits, a circuit for power supply for delivering a driving input to the IC may be composed of the IC. However, normally, a P type semiconductive substrate is used in the IC. Hence, in order to form PNP type transistors in the substrate, lateral PNP type transistors should be used. In this case, a wafer area of the IC having mounted therein with the circuit for power supply is large as compared with the case where NPN type transistors are used.
Furthermore, as the circuit for power supply for supplying a stabilized voltage to the IC even when the input signal voltage fluctuates in the circuit for the power supply for delivering the driving input to the IC, there may be proposed such a circuit as shown in FIG. 1.
In FIG. 1, applied to between input terminals 2A and 2B through the telephone line is a DC voltage V.sub.L having superposed thereon AC signals. The input terminal 2A is set at a high potential and the input terminal 2B is set at a low potential, and the latter is connected to a reference potential point (GND).
In a former stage of this circuit for power supply, a comparator 4 is provided and which has a pair of transistors 6 and 8 with an emitter common therebetween. A constant-current source 10 for determining the driving currents of the transistors 6 and 8 is connected to a portion between the emitter and the low potential side having the input terminal 2B formed thereon, i.e. a reference potential line.
Delivered to the base of the transistor 6 are a resistance from a resistor 12 connected to a portion between a high potential line and the reference potential line and the input voltage V.sub.L from a point connected to a constant current source 14, while, supplied to the base of the transistor 8 is a reference voltage V.sub.B for comparison to an output side of this circuit.
Furthermore, a first constant-output-current circuit 16 is connected to a portion between the collector of the transistor 6 and the high potential line, while, a second constant-output-current circuit 18 is connected to a portion between the collector of the transistor 8 and the high potential line. The first constant-output-current circuit 16 comprises transistors 20, 24 and resistors 26, 28. The base and collector of the transistor 20 and the base of the transistor 24 have a common connection therebetween. The transistors 20 and 24 form a current mirror circuit and an output therefrom is delivered to a first output-voltage-stabilizing circuit 30A. On the other hand, the second constant-output-current circuit 18 comprises transistors 32 and 34 and resistors 36 and 38, the transistors 32 and 34 form a current mirror circuit similarly to the first constant-output-current circuit 16. The transistor 34 is connected to a portion between the high potential line and the reference potential line with the collector being on the reference potential side, to thereby form a bypass circuit for flowing superfluous current to the reference potential line. The transistor 24 is composed of several tens of the transistor 20 arranged in an array, and the transistor 34 is also composed of several tens of the transistors 32 arranged in an array. This is because an area of the emitter of the transistor 24 is set to be several ten times that of the transistor 20, and an area of the emitter of the transistor 34 is set to be as much as several ten times that of the transistor 32, so that a saturation current value can be increased.
Formed on the collector of the transistor 24 is an output terminal 40 for taking out a stabilized output voltage Vcc, and a capacitor 42 for absorbing fluctuating components is connected to a portion between this output terminal 40 and the reference potential point i.e. ground. Furthermore, a second output-voltage-stabilizing circuit 30B is provided between the high potential line and the reference potential line. The high potential line is formed with an output terminal 44 for taking out a stabilized output voltage V.sub.B, and a capacitor 46 for absorbing fluctuating components is connected to a portion between this output terminal 44 and the reference potential point i.e. ground. The reference potential line is formed with a reference potential point terminal 48 associated with the input terminal 2B, and this reference potential point terminal 48 is connected to ground. The output voltage V.sub.B from the output-voltage-stabilizing circuit 30B is delivered to the base of the transistor 8 of the comparator 4 as a reference voltage.
As described above, in the circuit of this type when the input voltage V.sub.L as shown in FIG. 2(A) is given, the fluctuation in value thereof is more than the value V.sub.B of the voltage inverted by the operation of the comparator 4, whereby no distortion is caused to the waveform of AC signals. In FIG. 2(A), designated by V.sub.LO is an input voltage value saturated by the transistor 24.
Then, with the circuit for power supply as shown in FIG. 1, even when a value of the input voltage V.sub.L fluctuates and becomes lower than the voltages V.sub.B and V.sub.LO as hatchedly shown in FIG. 2(B), the first constant-output-current circuit 16 and the second constant-output-current circuit 18 are changed over to each other in response to an output from the comparator 4, whereby a stabilized voltage can be supplied, and moreover, the impedance is constant in value, so that no distortion is caused to the waveforms of AC signals.
However, in this circuit for power supply, the wafer area of the IC composing this circuit is disadvantageously enlarged. In other words, in this circuit for power supply, both the transistor 24 for supplying current to the first output-voltage-stabilizing circuit 30A and the transistor 34 as being the bypass circuit for flowing superfluous current to the reference potential side are arranged in arrays each consisting of several tens of PNP type transistor as shown in FIG. 3. In FIG. 3, designated at 201A, 201B and 201C are each an N layer of a PNP type transistor, and 202A and 202B are each a P layer, 204 an emitter and 205 a base. When the circuit is formed by a semiconductive integrated circuit including several PNP type transistors as described, the wafer area is disadvantageously enlarged.